1. Field of the Invention
This invention relates to a novel (meth)acrylic compound having a polymerizable oxetanyl group and a novel side-chain type liquid crystalline polymeric substance derived from the (meth)acrylic compound. This invention also relates to a liquid crystal film containing such a polymeric substance and a liquid crystal display mounted with such a liquid crystal film.
2. Description of the Prior Art
In recent years, as a result of the researches and developments which have been actively conducted to apply liquid crystal materials to optical purposes, a liquid crystal film produced by aligning a liquid crystal material has practically used for the purposes of providing a liquid crystal display with color compensation and wide viewing angle properties.
Various methods of forming a liquid crystal material into a film are known. For example, there is a method in which a liquid crystal film is produced by forming a liquid crystalline polymeric substance film over a substrate which can align the substance and then heating the film to a temperature equal to or higher than the glass transition temperature “Tg”, followed by rapid cooling of the film so as to fix the orientation of the liquid crystal, as disclosed in Japanese Patent Laid-Open Publication No. 3-9321. However, although this method is applicable to both main chain- and side chain-type liquid crystalline polymeric substances, it has problems that the process would be increased in load and eligible alignment substrates are limited when using a main-chain type liquid crystalline polymeric substance with a high Tg. Furthermore, when using a side-chain type liquid crystalline polymeric substance, this method has a problem in the heat resistance of the resulting liquid crystal film and a drawback that the orientation of the liquid crystal would be disordered at a temperature in the vicinity of the Tg.
Alternatively, as disclosed in Japanese Patent Laid-Open Publication, No. 8-21915, there is a method in which a liquid crystal film is produced by pouring a liquid crystalline low molecular weight substance having a reactive group into a liquid crystal cell provided with an alignment substrate and heating the cell to a temperature at which the substance exhibits a liquid crystal phase so as to be oriented in a liquid crystal orientation state, followed by fixing the orientation by polymerizing the substance with an external force such as light or heat. However, in this method, it is difficult to control the parameters of the resulting liquid crystal film because the liquid crystal to be oriented is a low molecular weight substance and thus the physical properties such as fluidity and birefringence are heavily dependent on temperature. Further, this method may often have a problem with the heat resistance of the resulting liquid crystalline film because the structure of the liquid crystal after polymerization becomes similar to that of a side chain-type liquid crystalline polymeric substance and thus the sufficiently high Tg can not be obtained.
As an alternative for polymerizing and fixing a liquid crystal orientation by an external force, there is a method in which a mixture of a liquid crystalline polymeric substance and a reactive low molecular weight substance is allowed to be oriented to a liquid crystal phase and subjected to an external force so as to be reacted and cured, as reported in Japanese Patent Laid-Open Publication No. 10-120640. However, because the reaction between the liquid crystalline polymeric substance and the reactive low molecular weight substance does not progress sufficiently, the Tg is not sufficiently raised, leading to a problem with the heat resistance of the resulting liquid crystal film.
In view of these problems, a method as disclosed in Japanese Patent Laid-Open Publication No. 2000-319527 is now considered to be the most effective in which method a reactive group is directly introduced into a liquid crystalline polymeric substance with a relatively low Tg, such as a side chain-type and then subjected to an external force such as light or heat to be cross-linked after orienting the reactive group to a liquid crystal state, thereby raising the Tg. However, this method has a problem that it is difficult to synthesize such a side chain-type liquid crystalline polymeric substance with a reactive group. For example, when the polymeric structure is constructed prior to the introduction of a reactive group, the amount thereof may be insufficient. On the other hand, in a method in which from a monomer having two reactive groups, a side chain-type liquid crystalline polymeric substance having a reactive group is synthesized by polymerizing one of the reactive group, it is necessary that the reactivity of the other reactive group must be maintained lower than that of the reactive group to be reacted, leading to a problem that the reactive group of the polymeric substance after being oriented to a liquid crystal phase tends to be insufficient.
The present invention was made in view of the foregoing problems and provides a novel compound which is suitable as the starting material of a side chain-type liquid crystalline polymeric substance having a reactive group with an excellent reactivity upon fixation of a liquid crystal orientation and a side chain-type liquid crystalline polymeric substance derived from such a compound. Furthermore, the present invention also provides a liquid crystal film with an excellent heat resistance which is produced from a liquid crystal material containing such a side chain-type liquid crystalline polymeric substance by fixing the orientation state of the liquid crystal material and a liquid crystal display mounted with the liquid crystal film.
After an extensive research and study made by the present inventors, they found a novel polymerizable compound having an oxetanyl group which can be used as the starting material of a side chain-type liquid crystalline polymeric substance which can be easily synthesized and is excellent in orientability and fixation of the orientation structure.